Wide angle imaging offers a different way to visualize the world. However, typical wide-angle images exhibit large depth of field (DOF). It is desired to provide depth of field effects for wide-angle images.
A conventional method acquires a wide field-of-view (FOV) lightfield using multiple sequential images by a moving camera with a narrow-angle lens or a wide-angle (fish-eye) lens. A translation stage or pan-tilt units can also be used. Wide angle panoramas and mosaics also require multiple images acquired by either rotating the camera around its optical axis, or along a path. These approaches require the scene to be static, and increase the acquisition time. A camera array with wide-angle lenses can avoid these problems. However, this is more expensive and less portable. Moreover, the multiple cameras need to be synchronized, and calibrated radiometrically and geometrically with respect to each other.
Lightfield Cameras
The concept of lightfield as a representation of all rays of light in free-space is well known. Lightfields acquired using camera arrays have been used for synthetic aperture imaging and digital refocusing.
Hand-held lightfield cameras trade spatial resolution for angular resolution. Those cameras either insert a micro-lens array or mask close to the sensor, or use an array of lens and prisms in front of the main lens. Those types of cameras offer only a limited FOV, which is determined by the main lens.
Different parameterizations such as 2-sphere and sphere-plane have been described for sampling lightfields over a sphere.
Catadioptric Imaging Systems
Catadioptric imaging systems (CIS), which include mirrors (cata) and lenses (dioptric), offer an alternative approach to increase the FOV in a single image. An omnidirectional sensor having a 360°×50° FOV uses four planar mirrors in a pyramidal configuration with four cameras.
An array of planar mirrors can be used to acquire a lightfield. A spherical mirror with a perspective camera leads to a non-single viewpoint or multi-perspective image when the camera is placed outside the sphere. Radial CIS, which use a camera looking through a single hollow rotationally symmetric mirror polished on the inside, can perform 3D reconstruction, generation of texture maps, and computation of the BRDF parameters. A single mirror results in a circular locus of virtual viewpoints.
Spherical mirror arrays have been used for acquiring incident lightfield for relighting, and for 3D reconstruction. Each sphere image is re-mapped into latitude and longitude format, and ray-tracing is used to determine intersection with the lightfield plane. In each sphere image, vertices in a 3D mesh can be manually marked to generate a simple 3D model, which is difficult to extend to natural scenes. Acquired images of multiple spheres can be tesselated for 3D estimation.
Although spherical lenses are easier to produce, the lenses cause spherical aberrations, and decrease image quality. Aplanatic spherical lenses have been used for high power microscope objectives and endoscope objectives, and refractive sphere models have been used for modeling rain drops by tracing rays.
None of the prior art methods, which use spherical mirrors, have been used for refocusing of a wide-angle image acquired by a single stationary camera.
Projections of Wide-Angle Images
Perspective projection of a wide-angle image shows severe distortions at the periphery of the image. To map a sphere to a plane, several projections, which trade off different types of distortions have been developed. However, a global projection cannot keep all straight lines straight and preserve the shapes of the objects.